Technical Field
The present invention relates to a semiconductor device, and in particular to a semiconductor device having a thin film resistive element formed on a field effect transistor through an insulating film.
Background Art
A switching power supply control IC, which is a semiconductor device, is specialized to control individual high breakdown voltage switching transistors. This IC, in an operational state, causes the high breakdown voltage switching transistors to operate to form their own power supply, but during start-up these transistors need to receive start-up current from the start-up circuit. The start-up circuit is ordinarily integrated on the same semiconductor substrate as the switching power supply control IC, which reduces the number of components and simplifies the power supply system.
The start-up current is the result of rectifying an input alternating current signal of AC 100 to 240V, and is supplied to the start-up circuit; thus, a normally-on device that is upstream from the start-up circuit requires a breakdown voltage of at least 450V. This normally-on device is formed monolithically with the switching power supply control IC, and therefore becomes a lateral junction field-effect transistor (JFET) having a high breakdown voltage. The current driving ability of this device determines the design specifications of the switching power supply device.
If the switching power supply device is unplugged from the outlet, and stops receiving voltage from the AC input, then the primary side input voltage will drop. If the switching power supply device continues to operate in this state, the ON time of the switching MOSFET will become long and produce heat. To prevent this problem, the switching power supply device has a brownout function that stops the switching operation of the power supply when the input voltage drops.
The methods to realize this brownout function can be generally categorized into external resistance dividing schemes and IC chip embedding schemes. In the IC chip embedding schemes, voltage dividing resistors having a high breakdown voltage are formed using the voltage-withstanding structure of a device (start-up device) with a high breakdown voltage.
When embedding inside the IC chip, the voltage-withstanding structure of the already-existing start-up device, which is the high breakdown voltage device, is used, and spiral-shaped resistors are added to a part of the structure. The resistors are arranged in a spiral shape such that the potential gradually decreases from the drain terminal disposed in the center of the start-up device, which has the highest potential, to along the periphery. The resistors are formed to the source and gate regions, which are arranged so as to surround the drain region. Therefore, by being integrated with the start-up device, it is possible to embed the resistors having the high breakdown voltage without providing a new voltage-withstanding structure (see Patent Document 1, for example).
When using a resistor voltage divider as the input voltage detection method, however, current consumption is always occurring in the resistor voltage divider while input voltage is being applied. This is an issue that occurs regardless of whether the resistor voltage divider is embedded in the semiconductor integrated circuit or not, and can ordinarily suppress power consumption by increasing the resistance of the entire resistor voltage divider.
When embedding this resistor voltage divider in an integrated circuit, however, the following two problems occur in increasing the resistance.
The first is that the area of the start-up terminal is enlarged in order to secure the length of the resistive element. The second is that, in order to increase the per unit length resistance, the impurity dosage is diluted, which greatly increases variations in the resistance. One method to solve the second problem is to add an adjustable circuit such as a trimmer, but this complicates the circuit configuration.